1. Field
One or more embodiments relate to a lithium transition metal phosphate including nano rod-like Fe2P crystals, a method of preparing the lithium transition metal phosphate, and a lithium secondary battery manufactured by using the lithium transition metal phosphate.
2. Description of the Related Art
Electronics, information, and communication industries have shown rapid development by manufacturing portable, small, light, and high-performance of electronic devices, and demands for a lithium secondary battery that may exhibit high capacity and high performance as a power source of electronic devices have increased. Furthermore, as electric vehicles (EVs) or hybrid electric vehicles (HEVs) have been put into practice, studies on lithium ion secondary batteries, which have high capacity and power and excellent stability, have been actively conducted.
As to cathode active materials for a lithium secondary battery, lithium-containing cobalt oxide (LiCoO2) is usually used, and additionally, the use of lithium-containing manganese oxides such as LiMnO2 with a lamellar crystal structure and LiMn2O4 with a spinel crystal structure and lithium-containing nickel oxides (LiNiO2) has also been considered.
LiCoO2 has various excellent physical properties such as cycle properties and thus is widely used, but is low in safety and high in cost due to resource limitations of cobalt as a raw material, and thus has limitation in large-scale use as a power source in the field such as electric vehicles. Since the crystal structure collapses during the charge-discharge, LiNiO2 has problems in that the battery capacity is severely reduced and thermal stability is low, and lithium manganese oxides such as LiMnO2 and LiMn2O4 are disadvantageous in poor cycle properties and the like.
Thus, lithium transition metal phosphate having an olivine structure, which is represented by lithium iron phosphate (LiFePO4), has drawn attention as a material which is satisfactory in terms of resources, costs, and stability.
Since LiFePO4 forms a tetrahedral structure while phosphor (P) and oxygen (O) in the crystal structure form a strong covalent bond, there is a great advantage in that LiFePO4 is considerably stable in terms of thermal and chemical aspects. LiFePO4 and FePO4 from which lithium is deintercalated have fundamentally the same structure, and thus are advantageous in that the two materials are structurally very stable even though lithium ions are deintercalated, and a decrease in capacity scarcely occurs even after a few hundred cycles due to the stabilization of the structure.
Even though there is such an advantage, the commercialization of lithium iron phosphate with an olivine structure has not been easily obtained. The reason is because there are disadvantages such as low electronic conductivity, low ion conductivity and production of impurities caused by side reactions.
Among the disadvantages, the low electronic conductivity may be overcome by a method of performing coating with a carbonaceous material (Patent Document 1), and the low ion conductivity may be overcome by a method of shortening a lithium ion diffusion path by maintaining the particle size to be ultrafine (Patent Document 2).
However, since the composition of lithium iron phosphate sensitively varies according to the preparation method thereof, it is difficult to overcome a disadvantage of obtaining a product containing impurities which does not have a desired composition or oxidation number of transition metals. As a result, material and battery characteristics deteriorate, and accordingly, productivity, reliability and economic efficiency are reduced.
For example, Patent Document 3 describes, as a Comparative Example, an example in which iron phosphide (Fe2P) as an impurity is produced due to the decomposition of LiFePO4 by heat treatment at high temperature (1,000° C.), and clearly describes that Fe2P does not have a function as a cathode active material for a lithium ion secondary battery.
During the process of preparing lithium iron phosphate (LiFePO4), impurities such as Fe2O3 and Li3Fe2(PO4)3 in addition to the aforementioned Fe2P may be additionally produced. Most of iron contained in the impurities is in the state where the oxidation number is +3, because iron electro-structurally tends to have an oxidation state of +3 rather than +2, and is easily oxidized in the process of calcination.
Thus, Patent Document 4 discloses an anion-deficient non-stoichiometric lithium transition metal polyacid compound, and describes that the production of iron with an oxidation state of +3 may be suppressed by controlling the ratio of phosphoric acid having an electric charge of −3 according to the method.
However, according to the present invention, it has been found that when the Fe2P crystal, which is publicly known as an impurity that reduces the performance of a battery in the related art, is included in the form of a nano rod in lithium transition metal phosphate, high rate capability and low-temperature properties of a lithium secondary battery including the crystal may be improved, thereby completing the present invention.